A filter element for separating solid particles from a gaseous or liquid medium is disclosed in Patent Document 1 (JP-A No. 61-502381) as a filter element for discharging to the outside only the air cleaned by removal of fine solid particles from air containing fine dust of an air conditioner or an exhaust gas containing fine solid particles of a combustion engine and as a filter element for discharging to the outside only the liquid cleaned by removal of fine solid particles from a liquid containing fine solid particles such as oil and wastewater.
The filter element can remove fine solid particles contained in a medium to be filtered by originally forming a fine primary filter layer, without having to depend on the formation of a filter layer for dust adhesion like a bag filter, by filling granular polyethylenes composed of a mixture of medium molecular weight polyethylenes and macromolecular weight polyethylenes in a mold, heating and sintering the granular polyethylenes to mold into a rigid filter base, and filling somewhat large pores of the filter base with a filler composed of polytetrafluoroethylene (PTFE) fine powders using a process such as a partial heat treatment. Further, the polytetrafluoroethylene has water and oil repellency, thus the filter can recover filtration ability by removing adhered dust (the aggregate of fine solid particles, etc.) through back washing.
However, as described above, the filter element, which has a construction obtained by heating and sintering granular polyethylenes composed of a mixture of medium molecular weight polyethylenes and macromolecular weight polyethylenes to mold into a filter base and filling pores of the filter base with a filler composed of polytetrafluoroethylene fine powders to form a fine filter layer, become disabled over time due to heat deterioration of the filter base or pulse fatigue deterioration caused by back washing. A filter element filled with the polytetrafluoroethylene fine powders, when it became disabled due to clogging and is discarded by burning, cannot be burnt because the polytetrafluoroethylene fine powders filled on the surface of the base are exposed to high heat and decomposed to a harmful low molecular weight organic fluoride gas such as ethylene tetrafluoride, propylene hexafluoride, and perfluorocyclobutane, thus having an adverse effect on the environment. Therefore, there is no other way than to bury it underground which is only transferring the pollution to the underground. Further, the specific gravity of polytetrafluoroethylene is 2.2, which is heavy, and very nonhydrophilic, thus the dispersion of the suspended fine powders is unstable, and when filling pores of the filter base with polytetrafluoroethylene fine powders, it is difficult for a coating solution having polytetrafluoroethylene fine powders dispersed therein to make an aqueous suspension comprising only a water as a medium thereof, thus the addition of ethyl alcohol or the like is required. In addition, during brush coating or spraying coating, the dispersion solution is required to coat while stirring so that irregularity in coating does not occur.
Further, the filter element in which polytetrafluoroethylene fine powders are used as a filler, requires further improvement in pressure loss (also referred to as “pressure drop”) and reducing production cost.
[Patent Document 1] JP 61-502381
An object of the present invention is to provide a filter element and a method for producing the same, which do not generate pollution at the time of a waste disposal, which can be easily produced by easy preparation of the coating solution for filling pores of the filter base, and which improve pressure loss and reduce production cost.